1. Field of the Invention
This invention relates to an ultrasonic imaging apparatus for obtaining information of the liquid flow, e.g., blood flow in a living body by utilizing the Doppler effect of ultrasonic waves, and displaying this information as a two-dimensional image.
2. Description of the Related Art
The ultrasonic imaging apparatus uses an ultrasonic Doppler process and a pulse reflection process in combination to generate blood flow data and tomographic image (B mode) data with a single ultrasonic probe and superimpose these data on each other. A superimposed image data is displayed, as a blood flow profile and a tomographic image, on the monitor.
The ultrasonic imaging is based on the following principle.
When a living body, in which blood is flowing, is irradiated with an ultrasonic beam, the center frequency fc thereof is distributed by flowing blood cells and receives a Doppler effect so that it is shifted by fd. That is, the ultrasonic echo resulting after the Doppler effect has a frequency f of f=fc+fd. The frequencies fc and fd are related as EQU fd=2v cos.multidot..theta./c.multidot.fc
where v is the blood velocity, .theta. is the angle between the ultrasonic beam and blood vessel, and c is the velocity of sound.
Thus, the blood velocity v can be obtained by detecting the Doppler shift frequency fd. When measuring the blood flow speed by utilizing the Doppler effect, the living body is irradiated with an ultrasonic pulse directed several times in a predetermined direction from an ultrasonic transducer. The echo pulses of the ultrasonic pulses from the living body, having received the Doppler effect, is received by an ultrasonic transducer to be successively converted to an echo signal. The echo signal is supplied to a phase detector to detect a Doppler shift signal. In this case, a Doppler shift signal is detected with respect to 256 sampling points in a raster direction of the ultrasonic pulses (i.e., depth direction of the living body under examination). The Doppler shift signal that is detected by each sample point is supplied to a frequency analyzer for frequency analysis and then converted by a DSC (digital scan converter) into a scanning signal to be displayed as a two-dimensional blood flow profile image.
When the Doppler shift signal is displayed as a blood flow profile image on the monitor, the mean blood velocity is displayed as an angle (+.pi. to -.pi.) or as a frequency (+fr/2 to -fr/2). The angle display (+.pi. to -.pi.) or frequency display (+fr/2 to -fr/2) is in a corresponding color (blue-black-red). fr is the ultrasonic pulse rate frequency.
In the blood flow display by the above color Doppler method, the Doppler shift signal is subjected to a frequency analysis calculation based on the first Fourier transformation (FFT) or color flow mapping (CFM). This calculation is that of the discrete value system referring to the rate frequency. Therefore, when the Doppler signal exceeding +fr/2 (or +.pi.) or -fr/2 (or -.pi.) is supplied to a processor, there occurs an aliasing phenomenon, i.e., a phenomenon, in which the signal of a level exceeding a threshold level of the display is inverted and the excess level portion of the signal is displayed in a color showing a blood flow direction opposite to the normal blood flow direction.
In the color display of the blood flow by the color Doppler method, Doppler image data that is extracted for improving the quality of the color display is interpolated. In the interpolation between two sample data, if interpolation data includes data corresponding to a black level, an image corresponding to an aliasing region is displayed as an image with a black frame. Such an image with a black frame is undesired from the diagnosis standpoint if it is displayed in a blood flow profile image.